In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as better control of contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
Efforts have been undertaken to enhance the quality of CT system support for interventional procedures, such as biopsies. Significant support issues include the amount of time necessary to scan and display an image, and the quality of the displayed image. Particularly, and with respect to interventional procedures, images are not displayed in "real time," i.e., a lag exists between data acquisition, or scanning, and image display. Furthermore, known CT fluoroscopy systems typically are configured to scan at a fixed location, and display only one image slice at a time. Since the interventional procedure only proceeds as fast as the CT system acquires and displays data, such interventional procedures are done on a step-by-step, or display-by-display basis, rather than a continuing basis.
To reduce lag time, helical scans can be used for imaging during interventional procedures. However, up to 10 seconds can be required to download each helical scan. Therefore, while scan time is reduced by using a helical scan, significant down time occurs between helical scans. Moreover, and because several helical scans are needed throughout an interventional procedure, the procedure is interrupted while each helical scan is downloaded.
It would be desirable to improve CT support for interventional procedures. Particularly, it would be desirable to acquire data, reconstruct such data and display an image for such data quickly enough to guide an interventional procedure. It also would be desirable to reduce any down time during an interventional procedure, and improve the image display for interventional procedures.